1. Field of the Invention
The present invention relates to a pulse modulator capable of generating a transmission pulse having an arbitrary shape in ultra wideband (hereinafter abbreviated as “UWB”) wireless communications.
The present invention also relates to a demodulation determining circuit which performs demodulation on the basis of a modulated pulse subjected to pulse position modulation (hereinafter abbreviated as “PPM”) on the transmitting side in UWB wireless communications.
2. Description of the Related Art
In a conventional wireless communication system or a UWB wireless communication system using a related art, transmission output frequency characteristics such as an occupied bandwidth, etc. are determined which are proportional to the rate of a transmission code or a diffusion code. This is done to attenuate frequency components of the transmission code or diffusion code by a band limit filter (see a patent document 1 (Japanese Unexamined Patent Publication No. Hei 10(1998)-65574) or 2 (Japanese Unexamined Patent Publication No. Hei 9(1997)-270733)). In the conventional wireless communications, communications are carried out using such a modulation signal.
In the conventional wireless communication system or the UWB wireless communication system using the related art, a carrier frequency is multiplied by a modulation component on the transmitting side to obtain a transmission output. On the receiving side, a frequency equivalent to the carrier frequency on the transmitting side is multiplied by a received input to extract the modulation component (refer to, for example, a non-patent document 1 (“Nikkei Electronics”, Nikkei Business Publications, Inc., Feb. 17, 2003, No. 841, pp. 108-109)).
In the UWB wireless communications, a method of bringing a transmit output waveform into pulsed form has been discussed to effectively use a communication band and ensure transmission power. Accordingly, the frequency characteristic is decided based on a pulse per se corresponding to one transmission output unit without the frequency characteristic being determined by the rate of the conventional transmission code or diffusion code or the like.
Therefore, there is a need to provide a mechanism for generating a pulse shape corresponding to a transmission code or diffusion code without causing the conventional transmission code or diffusion code to directly pass through the band limit filter, as a problem on the transmitting side. The conventional modulation circuit is not capable of generating such a pulsated signal.
On the other hand, there is known, as a problem on the receiving side, demodulation of a pulse position modulation system corresponding to a typical modulation system for UWB wireless communications, which shifts the generated position of a pulse according to information sources to thereby perform modulation. It is also feasible to take a pulse corresponding to a transmission output as electric power in the case of pulse position modulation and determine data according to whether the power is placed in a desired position. In the case of adoption of the conventional reception system, however, a received input is multiplied by a frequency equivalent to a carrier frequency on the receiving side to thereby make conversion into a baseband signal, and signal demodulation is carried out by a high resolution sampling-based processing system for determining a pulse or a Rake reception system which absorbs a time delay of an incoming wave due to a multipath, thereby bringing about a receiving device large in circuit scale.